The characters can be given a back story, or they may just serve as a way to move the plot along..

The plot is one of the main drivers in the thriller, The characters must be a quest to help others by their actions.

There may be a mystery involved, but it's not a requirement of the subgenre.

Most definitely. A thriller need to have suspense, the feeling that something bad is going to happen. It helps activate the characters to solve the problem and save the world.

It goes without saying.

There can definitely be strange elements in a thriller. New weapons, different technologies, anything that may threaten the world. And the villain should be larger than life.

Violence in a part of any thriller. The violence must be sufficient to make the stakes high for the hero and the people involved.

It wouldn't be a thriller without action. Most thrillers have higher page counts, which in turn means more events must occur to keep things moving swiftly.

Obviously, these fall into the category of thriller most of all. Thriller need suspense to keep the reader turning those pages. The stakes must be high and the results of failing must be intolerable. While there may be mystery elements, as perhaps the main characters don't know who they are fighting against, a mystery is not required for a thriller.